1. Field of the Invention
The invention relates to a pressure tapping type ladle for transferring molten metal and a molten-metal tapping method for use in transferring and supplying molten metal, such as molten aluminum, to a molten-metal holding furnace placed in a molten-metal casting facility.
2. Description of the Related Art
In manufacturing castings of aluminum or aluminum alloy by a die casting method, the casting is usually performed at a plant equipped with many die-casting machines in order to increase productivity. Molten metal is supplied to a die-casting machine by transferring molten metal from a molten-metal holding furnace to a molten-metal ladle, and then supplying molten metal from the molten-metal ladle. Since a predetermined amount of molten metal always needs to be retained in a molten-metal holding furnace, molten metal obtained from a melting furnace located in the plant or molten metal brought from the outside of a plant is continuously supplied thereto so that a predetermined amount of molten metal may be maintained.
FIGS. 14A and 14B are cross-sectional views of one example of a conventional pressure tapping molten-metal transferring ladle. FIG. 14A shows an entire molten-metal transferring ladle and FIG. 14B is a view when the working cover is open. The molten-metal transferring ladle shown in FIGS. 14A and 14B comprises: a ladle body 101 for holding molten metal; a top cover 102 covering the ladle body; an openable working cover 103 provided in the top cover 102; a gas inlet 104 that is provided in the working cover 103 and pressurizes the surface of the molten metal (molten-metal surface) in the ladle; and a molten-metal tapping portion 105 provided in the ladle body 101.
The working cover 103 covers an opening 111 for use in pouring molten metal to the ladle body 101, removing scum (aluminum oxide, etc.) formed on the molten-metal surface, measuring the molten-metal temperature, etc. The outer surfaces of the ladle body 101, the top cover 102, and the working cover 103 are usually covered with steel shells 107, 108, and 109, respectively. The insides of the ladle body 101, the top cover 102, and the working cover 103 are laminated with a fireproof material 110. Furthermore, in order to raise the heat insulating property, a heat insulating material, etc., may be laminated between the fireproof material 110 and each of the outer steel shells 107, 108, and 109. In order to control the molten-metal tapping rate or to stop the pressurizing, a gas outlet 113 for discharging introduced gas is provided.
For tapping molten metal, a gas, such as air, is introduced through an opening 112 from the gas inlet 104 to pressurize the molten-metal surface, thereby supplying molten metal to a molten-metal holding furnace from a tap hole 106. In the conventional tapping method of inclining a ladle, and then pouring molten metal to a holding furnace while controlling the molten-metal tapping rate, advanced skill is required and the workload is increased for conducting the same procedure at a plurality of molten-metal holding furnaces. By tapping molten metal by pressurizing a molten-metal surface as described above, such disadvantageous operations can be eliminated.
As described above, molten metal may be supplied to a molten-metal holding furnace located inside a casting plant from a metal-melting plant located outside the plant. In this case, a molten-metal transferring ladle containing the molten metal is transported by a truck or like conveyance. When a truck, etc., travels on a public road, the molten-metal surface may shake greatly due to the roughness of the road surface or the curves at street corners. This may accidentally splash molten metal, which will adhere to the inner surface of the top cover 102 or the working cover 103.
FIGS. 14A and 14B show a molten-metal transferring ladle in which the gas inlet 104 is formed in the working cover 103 (e.g., Japanese Patent No. 3323489). In the prior-art molten-metal transferring ladle, in which the gas inlet 104 is not formed in the working cover 103, the gas inlet 104 is provided mainly in the top cover 102. In the case of the molten-metal transferring ladle shown in FIGS. 14A and 14B, since an opening 112 of the gas inlet 104 is formed in the working cover 103, which can be provided at a longer distance from the molten-metal surface, the frequency with which the opening is clogged with molten metal or molten-metal splashes is reduced compared with the case where the gas inlet 104 is formed in the top cover 102.
Forming the gas inlet 104 in the working cover 103 reduces the frequency of clogging of the opening of the gas inlet due to molten-metal splashes to some extent. However, it cannot be said that the clogging of the opening is completely prevented considering the fact that clogging may occur due to road surface conditions, the type of conveyance, such as a truck, the amount of molten metal, etc. The clogging of the opening 112 hinders the tapping of molten metal, and in the worst case, the clogging makes it difficult to pour the molten metal.